It looks like they have been on the food for about 2 weeks so far. This is usually enough time for them to have set the habit of eating the new food so that they will now stick with it. There would be nothing wrong with you giving them a treat in the morning and when you get home from work each day. Only use a canned food that does not contain any plant material in the ingredients list on the back label or something like a freeze dried meat. We also have treats available.
We always tell clients that cats can't count so quantity is not as important as the act of giving. I would split the 3oz can into 6 treats. If you feed a spoonful two times a day the can would last 3 days. This is a good way to get a head count twice a day (sometimes cats get locked in closets etc.) and it is a great way to bond with them. It is also not enough food for them to go off their main food.
Other suggestions would be to pick the cat up and take them to another room to play or be brushed. Try new things and they will respond as they learn that new things are also fun. Another good bonding behavior is head bump your cat. Sometimes it is a bump and other times it is more of a rub, in any case most cats respond well to this.
Be patient and not only will you get an even better relationship with your kitties, but they will feel great and lead a healthy life.